The problem of how to dispose of municipal waste is becoming serious in many cities since the amount of municipal waste is rapidly increasing in every city.
Some of the constituents of the waste are recovered by the method and apparatus disclosed in U.S. Pat. Nos. 3,973,735 and 4,076,177. However, some part of the waste is usually incinerated for disposal which may result in loss of usable resources.
If organic materials are thermally decomposed, pyrolysis gas may be recovered therefrom. To such end, a two-bed type of pyrolysis apparatus such as is employed in the petrochemical, coal-chemical or the like processes has been utilized. However, the two-bed thermal reactor of the prior art was originally designed for relatively uniform materials such as petroleum or coal rather than a mixture of types of material. Thus, special consideration should be given to treating municipal waste, which contains a mixture of several kinds of materials including solids and non-organic materials, in the two-bed pyrolysis apparatus.
A two-bed pyrolysis apparatus generally comprises a pyrolysis fluidized bed reactor where endothermic decomposition is performed to produce pyrolysis gas and a regenerator or combustion fluidized reactor where primarily an exothermic reaction is performed with respect to char, oil and tar produced in the pyrolysis reactor and introduced therein. In the combustion reactor, pyrolysis gas generated in the pyrolysis reactor may be introduced for aiding regeneration of sand in case the amount of char, oil and tar to be burnt therein is insufficient and, therefore, variation in the amount of exhaust gas from the regenerator is made relatively small. However, in the pyrolysis reactor, the amount of pyrolysis gas generated as well as the free board pressure of the pyrolysis reactor vary due to the fact that the type and size of the constituents of waste to be decomposed and their water content vary widely whereby, as a consequence, stable circulation of fluidized medium or sand may be obstructed.
On the other hand, the composition and the amount of generated pyrolysis gas are greatly influenced and are subjected to variation by the pyrolyzing temperature. It is difficult to keep the pyrolyzing temperature constant if the composition, water content, etc. of the material to be pyrolyzed vary.
Therefore, it has been generally experienced that the composition and the amount of pyrolysis gas generated in the conventional two-bed pyrolysis apparatus are not maintained constant. Variation in the composition of the generated gas naturally leads to inconvenience in its use since regulation of the nozzle size of the burner or adjustment of other elements is required to cope with such variation.
Also, continuous operation of the two-bed pyrolyzing apparatus is sometimes disturbed due to blocking or blowing through in a passage for circulating fluidized medium or sand between two reactors. Such blocking or possibility of blowing through is enhanced when the municipal waste is processed in the two bed pyrolysis apparatus since the waste usually contains several articles of foreign material such as solids and non-organic materials which may not be incinerated and may become clinkers.